Cookies and Sessions

A PHP session variable is used to store information about, or change settings for a user session. Session variables hold information about one single user, and are available to all pages in one application.

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PHP Session Variables

When you are working with an application, you open it, do some changes and then you close it. This is much like a Session. The computer knows who you are. It knows when you start the application and when you end. But on the internet there is one problem: the web server does not know who you are and what you do because the HTTP address doesn't maintain state.

A PHP session solves this problem by allowing you to store user information on the server for later use (i.e. username, shopping items, etc). However, session information is temporary and will be deleted after the user has left the website. If you need a permanent storage you may want to store the data in a database.

Sessions work by creating a unique id (UID) for each visitor and store variables based on this UID. The UID is either stored in a cookie or is propagated in the URL.

Understanding Cookies and Sessions

Cookies and Sessions are inextricably intertwined, and fundamental to creating rich, interactive internet applications.

What we will be covering in this tutorial:

• What is a "cookie"
• What is a "session"
• Setting and reading cookies
• Using sessions
• Troubleshooting
• Further reading

What is a "Cookie"?

A cookie is a small piece of text stored on a user's computer by their browser. Common uses for cookies are authentication, storing of site preferences, shopping cart items, and server session identification.

Each time the users' web browser interacts with a web server it will pass the cookie information to the web server. Only the cookies stored by the browser that relate to the domain in the requested URL will be sent to the server. This means that cookies that relate to www.example.com will not be sent to www.exampledomain.com.

In essence, a cookie is a great way of linking one page to the next for a user's interaction with a web site or web application.

What is a "Session"?

A session can be defined as a server-side storage of information that is desired to persist throughout the user's interaction with the web site or web application. 

Instead of storing large and constantly changing information via cookies in the user's browser, only a unique identifier is stored on the client side (called a "session id"). This session id is passed to the web server every time the browser makes an HTTP request (ie a page link or AJAX request). The web application pairs this session id with it's internal database and retrieves the stored variables for use by the requested page.